Cool ! Everything seems to work as specified. Nevertheless the rules to allow specifying one track as locked seem a bit out of nowhere…
Ok I understand that whatever the mode (2x2 or 6x6) you allow n-1 tracks to be locked. Totally legit, after all what’s the point if everything is locked.
But when it comes to which track can we lock, this rule you mention makes particularly no sense:
Question is why the first consecutive tracks ?? Sounds really like an implementation limitation more than a technical/physical limitation. Actually if the system is able to lock one (or more ) track, what is the justification to force me to use this one more than this one ?
Why am I insisting on that ? It is related to the flow of a live performance.
If I want to record something using my DAW, I would probably (although not forced to) record first the drums, then bass, then guitars and voices… But actually the order I record makes no difference. People will only listen to the resulting mix.
When I perform live, it’s a bit different, I will probably not perform in that order because the audience would probably be gone before I end recording the drums 
, I would probably start by recording a guitar to establish the tune (a track I don’t want to lock as changing along song parts), then record some tracks around (percussive guitar part, pseudo-bass using the guitar…) and will want to lock the latters.
So with the rules you have defined this is still possible I just need to record my guitar as track 2 and then the track/overdubs I want to lock as track 1…
So ok, it just means I need to lose the habit to record the track 1 first… which is not really intuitive, but still doable as recording the track 2 is directly accessible in this 2X2 mode.
Now if I talk 6x6, this becomes a bit weirder… If I take the same use case, it means that I have to switch to a subsequent track beforehand, and unfortunately, recording to a subsequent track, is not directly accessible in 6x6… For me, that renders that flow far too complex, if not unusable.
As a conclusion, for me everything is fine except that you have definitely to find a way to get rid of the limitation of the first consecutive tracks only to be eligible to the locking mechanism. I understand you have probably decided for this behavior because of other impacts I do not clearly see. Please do not hesitate to share them with us. Pretty sure we can come to a solution.